Organic semiconductor materials have become a cornerstone in the development of organic field-effect transistors (OFETs), offering unique advantages such as mechanical flexibility, low-temperature processing, and tunable electronic properties. The active layer in OFETs, which governs charge transport, is typically composed of small molecules or conjugated polymers. The performance of these materials is dictated by their molecular structure, crystallinity, and processing methods, all of which influence charge carrier mobility and environmental stability.
Small molecules like pentacene and rubrene are among the most studied organic semiconductors for OFETs. Pentacene, a polycyclic aromatic hydrocarbon, exhibits high hole mobility due to its planar structure and strong π-π stacking, which facilitates efficient charge transport. Single-crystal pentacene OFETs have demonstrated mobilities exceeding 1 cm²/Vs, while thin-film devices typically achieve values around 0.1 to 0.5 cm²/Vs. The crystallinity of pentacene films is highly dependent on deposition conditions; vacuum evaporation often yields superior film quality compared to solution processing due to better molecular ordering. Rubrene, another high-performance small molecule, shows even higher mobilities in single-crystal form, reaching up to 20 cm²/Vs, attributed to its herringbone packing motif that enhances orbital overlap.
Conjugated polymers, such as poly(3-hexylthiophene) (P3HT) and diketopyrrolopyrrole (DPP)-based polymers, offer the advantage of solution processability, making them suitable for large-area, low-cost fabrication. P3HT, a benchmark polythiophene derivative, exhibits mobilities in the range of 0.01 to 0.1 cm²/Vs. The regioregularity of P3HT plays a critical role in its performance; highly regioregular chains promote self-assembly into well-ordered lamellar structures with improved charge transport. DPP-based polymers, on the other hand, have emerged as high-mobility materials due to their strong electron-accepting character and planar backbone, enabling mobilities exceeding 1 cm²/Vs in optimized devices. The extended π-conjugation and rigid structure of DPP polymers enhance intermolecular interactions, leading to improved crystallinity and charge delocalization.
The crystallinity of organic semiconductors is a key determinant of charge transport. Small molecules often form highly ordered crystalline domains when deposited via vacuum evaporation, minimizing grain boundaries that act as charge traps. In contrast, solution-processed films, particularly those of conjugated polymers, tend to exhibit semicrystalline morphology with a mix of ordered and disordered regions. Techniques such as solvent annealing, thermal annealing, and blade coating have been employed to enhance polymer crystallinity. For instance, slow drying of P3HT solutions promotes the formation of nanofibrillar structures, significantly improving mobility.
Molecular design strategies are critical for optimizing both mobility and environmental stability. Incorporating alkyl side chains in polymers improves solubility but can hinder charge transport if excessive. Balancing side-chain length and branching is essential to maintain processability without sacrificing crystallinity. For small molecules, functionalization with electron-withdrawing or donating groups can tune energy levels, influencing charge injection and air stability. For example, fluorination of pentacene derivatives lowers the highest occupied molecular orbital (HOMO) level, reducing susceptibility to oxidative degradation.
Environmental stability remains a challenge for organic semiconductors, particularly for materials with high-lying HOMO levels that are prone to oxidation. Encapsulation techniques and the use of stable electrode materials mitigate degradation, but intrinsic stability must be addressed through molecular engineering. Hybrid systems, such as polymer-small molecule blends, have shown promise in combining the high mobility of small molecules with the mechanical robustness of polymers.
Processing techniques significantly impact device performance. Vacuum deposition is preferred for small molecules due to precise control over film morphology, while solution processing is advantageous for polymers, enabling roll-to-roll manufacturing. Recent advances in inkjet printing and slot-die coating have improved the reproducibility of solution-processed OFETs. The choice of solvent, concentration, and deposition speed also affects film uniformity and molecular packing.
In summary, the development of high-performance OFETs relies on a deep understanding of structure-property relationships in organic semiconductors. Small molecules excel in mobility but face challenges in solution processing, whereas conjugated polymers offer processability with slightly lower performance. Advances in molecular design, crystallinity control, and processing techniques continue to push the boundaries of organic electronics, paving the way for flexible, low-cost, and efficient devices.